Corrosion Resistance of The Bearing Steel 67SiMnCr6-6-4 with Nanobainitic Structure

Abstract

The paper describes a comparative study of the corrosion resistance of bearing steel 67SiMnCr6-6-4 after two kinds of nanostructuring treatments and two kinds of conventional quenching and tempering treatments. The nanostructuring treatment consisted of austempering with an isothermal quenching at 240• C and 300• C. The conventional heat treatment consisted on quenching and tempering at 350• C for 1 h and quenching and tempering at 550• C for 1 h. Time and temperature of tempering was chosen so that the hardness of both samples (nanostructured as well as quenched and tempered) was similar. The microstructure of steel after each heat treatment was described with the use of transmission electron microscopy (TEM). It was shown, that the austempering conducted at 240• C produced homogenous nanobainitic structure consisting of carbide-free bainite plates with nanometric thickness separated by the layers of retained austenite. The austempering at 300• C produced a sub-micrometric carbide-free bainite with retained austenite in form of layers and small blocks. The conventional heat treatments led to a tempered martensite microstructure. The corrosion resistance study was carried out in Na 2 SO 4 acidic and neutral environment using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. The corrosion resistance of nanostructured steel samples were compared to the steel samples with tempered martensite. The obtained results indicate, that the corrosion resistance of bearing steel with nanobainitic structure is similar to steel with tempered martensite in both acidic and neutral environment. This means that the high density of intercrystalline boundaries in nanobinite does not deteriorate the corrosion properties of the bearing steel.Keywords: bearing steel, austempering, nanocrystalline structure, corrosion resistance, nanobainite W pracy przedstawiono porównawcze badania odporności korozyjnej łożyskowej stali 67SiMnCr6-6-4 poddanej dwóm typom procesów nanostrukturyzacji oraz dwóm typom konwencjonalnych obróbek hartowania i odpuszczania. Obróbka nanostrukturyzacji polegała na hartowaniu z przystankiem izotermicznym w temperaturze 240• C oraz 300 • C. Konwencjonalna obróbka cieplna obejmowała hartowanie i odpuszczanie w temperaturze 350• C przez 1 h oraz hartowanie i odpuszczanie w temperaturze 550• C przez 1 h. Czas i temperatura odpuszczania dobrane były tak, aby twardość próbek (po nanostrukturyzacji oraz hartowaniu i odpuszczaniu) była zbliżona. Mikrostruktura stali po różnych obróbkach cieplnych określona była przy użyciu transmisyjnego mikroskopu elektronowego. Wykazano, że hartowanie izotermiczne w temperaturze 240• C pozwoliło na wytworzenie jednorodnej struktury nanobainitycznej, zbudowanej z płytek bezwęglikowego bainitu, porozdzielanych warstwami austenitu szczątkowego. Podczas hartowania izotermicznego w temperaturze 300• C wytworzono bainit bezwęgli-kowy o submikronowej wielkości ziaren z austenitem szczątkowym w postaci warstw oraz niewielkich bloków. W wyniku konwencjonalnych obróbek hart...

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“…A highly refined structure is generally achieved by a low-temperature transformation in the range of 125-350 • C and simultaneously through careful alloy design of high-carbon and high-silicon steels [2]. Typical nanocrystalline high-carbon bainitic steels are characterized by poor overall in-use properties including weldability [3], corrosion resistance [4,5], and limited resistance to tempering processes [6], which are one of the major causes for their restricted industrialization. The concept of enhancing in-use properties by improving the thermal stability of bainitic steels has been extensively described in previous work [7].…”

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“…The range of research on advanced high strength steels, including nanostructured steels, which encapsulates inter alia their functioning in corrosive environments, 28 where hydrogen embrittlement is an important issue, 29 continues to grow. Another important matter raised at the conference was the practical applications of nanostructured steels 30,31 and cast iron 27 as well as new welding methods 32–34 and post-welding treatments to regenerate properties.…”